1. Field of the Invention
The useful process disclosed hereinafter is directed to making thermoplastic polymer fibers suitable for mixing with cellulosic pulp. The process is also directed to the mixing of the fibers with cellulosic pulp to make paper having improved properties. One example of a useful cellulosic pulp is bleached kraft paper.
2. Background Art
U.S. Pat. No. 3,770,856 discloses the production of fine fibrous structures by flashing an aqueous emulsion containing a polymer and solvent from a higher pressure and temperature zone to a lower pressure and temperature zone. The resulting structure is macerated in a mixer whereby obtained are fine flat fibers in a fibrillar state of an average width of from 5 to 10 microns and length of from 3 to 5 mm. The resulting fibers can be pressed into a synthetic paper.
British Pat. No. 1,221,488 discloses a process for the production of yarn which involves extruding a blend of polyethylene and blowing agent so as to produce an extrudate of foamed polyethylene. The latter is drawn so that it becomes orientated essentially in the direction of extrusion. The drawn foamed polyethylene is subjected to forces such that the walls of the foam are broken down. The resulting extrudate is a three-dimensional structure of interconnected fiber elements.
U.S. Pat. No. 3,634,564 discloses a process for the manufacture of fibrillated foamed films involving mixing a thermoplastic polymer with a blowing agent and extruding the mixture into a foamed polymer film and thereafter stretching the foamed film uniaxially. The stretching at an elevated temperature causes the voids of the foam to split.
The last two aforementioned patents disclose that the extrudate is orientated. Orientation refers to a process wherein the crystalline structure in polymeric materials are placed in alignment so as to produce a highly uniform structure. It is believed that orientation causes the axes of the molecules of the polymer to more generally line up in the same direction. Generally orientation is obtained by stretching (or pulling) the polymer while its temperature is below its melting point but above its transition temperature. But orientated extrudate, upon cutting and/or shearing into short fibers, does not yield suitable fibers for mixing with cellulosic pulp.
U.S. Pat. No. 3,954,928, discloses a process for the preparation of fibrillated extrudate by extruding a molten thermoplastic resin containing a foaming substance through a die. The extrudate is quenched almost as it leaves the die to a temperature below the resin's glass transition temperature. The resin can be a blend of polystyrene and a polyolefin e.g., polyethylene but the latter is present in an amount of at most 40% and preferably 30% or less by weight based on the blend.
Surprisingly applicant has found that a polyolefin by itself can be processed to fibers suitable for blending with cellulosic pulp. Equally surprisingly is that applicant has found that a mixture of polystyrene and polyethylene or polypropylene in which the mixture contains more than 40 weight % of the polyolefin is equally suitable for blending with cellulosic pulps.